1. Field of the Invention
The present invention relates to a process and a device for producing sterile-packed bone cements. It relates in particular to a process in which a cement powder component is subjected to gas sterilization in the containers that are used for packing.
2. Description of the Related Art
The natural joints in the human body are often subject to degenerative changes resulting, inter alia, in some cases from defects following surgical interventions occasioned by disease or accidents. If these defects have progressed too far, are irreversible or can no longer be treated, it becomes necessary to replace the natural joints or bones by corresponding implants. Examples of bone replacement materials that may be mentioned are shaped implants of a wide variety of kinds, or bone-connecting elements in the form, for instance, of medullary nails, bone screws or osteosynthetic plates. For implantation, these bone replacement materials are anchored in the natural bone using bone cements.
Customary bone cements are composed of a solid component, which consists of a finely divided polymer of acrylic and/or methacrylic esters and of further additives, such as polymerization catalysts, together, if desired, with X-ray contrast media, fillers and colorants, and of a liquid component, which consists of an acrylic and/or methacrylic ester monomer and of further additives such as polymerization accelerators and stabilizers. The polymer powder component of the cement consists preferably of granular particles with a spherical shape. The particle size preferably lies within a narrow range or is substantially uniform.
For use, the solid component and the liquid component are combined to form a liquid to semisolid paste. This paste is optionally brought in to a desired shape or is applied at the implantation site of a prosthesis in order to cement it in. The composition cures by means of the polymerization reaction induced with the mixing of the components. The bone cement is judiciously provided in a form which brings together separate containers, with matched amounts of the two components, in a single pack unit.
There are various possibilities for the separate packing of the individual components for preparing bone cement. For example, the sterile polymer powder can be packed into polyethylene pouches, while the liquid component is subjected to sterile filtration and dispensed, for example, into glass ampoules.
There are also packs which consist of flexible films and which possess two or more separate chambers separated from one another by removable seals. To mix the ready-to-use bone cement, the seal is removed or opened and the components are mixed homogeneously by kneading the flexible pouches. A system of this kind is described, for example, in the document WO 94/16951.
Also known are packs which are constructed as just described but which additionally possess a vacuum reservoir, attached to the powder container, and where the powder is packed under vacuum. The vacuum pack has the advantage that when the cement is mixed less air is incorporated into the bone cement, which has a positive effect on the porosity of the cement. Furthermore, the presence of a vacuum reservoir makes the full mixing of the polymer powder with the monomer, after the sealing device has been opened, much simpler and more effective, since the reduced pressure in the powder container draws the monomer right in under suction.
The individual components for preparing the bone cement must of course be packaged in a sterile manner. The customary sterilization techniques include chemical sterilization techniques, such as gassing with ethylene oxide, and sterilization by means of high-energy radiation, usually .gamma. radiation or .beta. radiation.
In the case of the closed cementing systems, and especially in the case of vacuum-packed cement powders, the individual components have, to date, either been sterilized by one of the above sterilization techniques, prior to dispensing, and then dispensed into the respective containers by way of sterile filters--in other words, aseptic dispensing is practiced--or the components have been sterilized within the packs by irradiation.
It is known, however, that high-energy radiation has a great effect on the properties of the bone cement. In particular, the viscosity is altered for the worse, and the shelf life has also been found, by our own investigations, to suffer a decrease.
It is known, however, that high-energy radiation has a great effect on the properties of the bone cement. In particular, the viscosity is altered for the worse, and the shelf life has also been found, by our own investigations, to suffer a decrease.